This invention relates to photopolymerization processes directed to the production of optical devices.
Since the advent of extensive research involving both analysis and experimental work on integrated optical devices and systems for communication and data processing, procedures for developing devices having a high resolution and index of modulation of a desired pattern of index of refraction have become increasingly desirable.
For example, the use of distributed feedback, sometimes called integrated feedback, in lasers is one use of such a pattern of index of refraction. Such a laser can be operated without the need for reflectors external to or applied at the ends of the optical gain medium. Other possible uses include frequency selective filters and related channel-dropping and channel-adding apparatuses.
Much prior work has been pursued with photosensitive monomers and polymers to produce desired density or index of refraction modulations in optical devices. The prior proposals which achieved the highest index modulation have employed polymers doped with a monomer or another photo-activated component. These materials are not suitable for use in a closed cavity, or in an elongated restricted space open only at the ends, because of the high viscosity of the initial polymer solution, or because they require the evaporation from the device of some unreacted monomer or other component. Furthermore, the prior photoinduced reactions typically result in a substantial shrinkage; and in a closed cavity with a constant volume this can lead to cracking or the formation of voids.
Other prior proposals have employed only monomers, which are of sufficiently low viscosity to flow into confined spaces, and which do not require the evaporation of unreacted components. However, with these materials the achievable modulation of refractive index is insufficient for many applications, and the shrinkage is such that they cannot be used in closed constant-volume cavities.
An example of such a closed constant-volume cavity is a distributed feedback laser in which the index pattern is in a thin layer between a central active medium and an outer tubular jacket. The central core and the jacket are preformed and spaced, and provide very great impediment to viscous flow because of their elongation and very small lateral spacings. Even if the ends are not sealed before the index pattern is written, the increasing viscosity of the material during polymerization would prevent shrinkage in the longitudinal direction (if such shrinkage could take place it would distort the desired index pattern), so that a substantially constant-volume process appears desirable to prevent cracking and formation of voids.
There are many other applications of photopolymerization in which similar problems of insufficient index change or void formation exist.